Why wouldn't they be able to? A company can take down or put up any offer they want whenever they want. Once they've taken your money you've entered in to a contract with them, but before that they can do whatever they want.
Edit: I'm certainly not a legal professional, so my opinion is not the law. On most print advertisements, I see advertisers give a disclaimer such as "We are not required to offer any mistakes in printing etc." This covers them legally since they need to advertise honestly.